(1) Field of the Invention
The present invention relates to an optical amplifying apparatus, an optical output controlling method by the optical amplifying apparatus, and an optical transmitting apparatus suitable for use to control an optical amplifier in an optical communication system.
(2) Description of Related Art
An optical fiber amplifier using an optical fiber doped with mainly a rare earth element such as erbium (Er) or the like has features of high gain, low noise, etc., which plays an important role in an optical transmission system as an optical amplifier amplifying an optical signal without converting the optical signal into an electric signal.
In the optical transmitting system, the optical amplifier is required an optical output constant control to keep an optical output level constant in order to absorb a difference in level of optical inputs to the optical amplifier due to a difference in transmission path length.
An optical amplifying apparatus performing the optical output constant control will be now described with reference to FIG. 9. FIG. 9 is a diagram schematically showing a structure of the above optical amplifying apparatus.
The optical amplifying apparatus has, as shown in FIG. 9, optical/electric converting units 101 and 108, an input disconnection detecting unit 102, an operation sequence unit 103, a pumping source 104, a pumping source driving unit 105, an operation change-over switch 106, an optical output constant control unit 107, a first demultiplexer 109, isolators 110 and 113, an erbium-doped optical fiber 111 (hereinafter referred as EDF 111), a multiplexer 112, and a second demultiplexer 114.
The first demultiplexer 109 is configured with an optical fiber coupler, which sends an optical signal inputted to the optical amplifying apparatus to the isolator 110, while demultiplexing a part of the optical signal and sending the optical signal to the optical/electric converting unit 101. The optical/electric converting unit 101 converts the optical signal sent from the first demultiplexer 109 into an electric signal (voltage signal). In other words, the optical/electric converting unit 101 converts the optical signal demultiplexed by the first demultiplexer 109 into a voltage signal proportional to an optical input level of the optical signal, and sends the voltage signal as an input monitor signal to the input disconnection detecting unit 102.
The input disconnection detecting unit 102 compares the input monitor signal sent from the optical/electric converting unit 101 with a reference value set in advance in the input disconnection detecting unit 102, determines that input light is disconnected when the input monitor signal drops below the reference value, and notifies the operation sequence unit 103 that the input light is disconnected.
The operation sequence unit 103 controls an operation of the operation change-over switch 106. The operation sequence unit 103 controls the operation change-over switch 106 to stop the optical output constant control when notified of disconnection of the input light from the input disconnection detecting unit 102.
A power-on/recovery detecting unit not shown detecting that power is on or that the input light signal is recovered is connected to the operation sequence unit 103. When the power-on/recovery detecting unit detects that power is on or that the input optical signal is recovered, the operation sequence unit 103 is notified of that.
Namely, the operation sequence unit 103 controls the operation change-over switch 106 to perform the optical output constant control when the power-on/recovery detecting unit detects that power is on or that the input light is recovered.
An end of the operation change-over switch 106 is connected to the pumping source driving unit 105. The other end of the operation change-over switch 106 is switchabley connected to either the optical output constant control unit 107 or ground, so that the operation change-over switch 106 is selectively switched under a control of the above operation sequence unit 103. The other end of the operation change-over switch 106 is connected to the optical output constant control unit 107 when the inputted optical signal is in a steady state condition, while being grounded when the inputted optical signal is disconnected.
The pumping source driving unit 105 drives the pumping source 104 to generate pump light, which is controlled by the optical output constant control unit 107. The pumping source 104 is configured with a light emitting element such as a semiconductor laser diode or the like, which supplies an energy of the pumping source 104 to the EDF 111 to amplify inputted optical signal.
The multiplexer 112 is configured with an optical fiber coupler or the like, which outputs pump light outputted from the pumping source 104 to the EDF 111, while outputting optical signal amplified by the EDF 111 to the isolator 113. The isolators 110 and 113 are connected to the both ends of the EDF 111, which are served to prevent resonance of the optical amplifier caused by that amplified light amplified by the EDF 111 is fed back to the EDF 111.
The second demultiplexer 114 is configured with an optical fiber coupler or the like, which is demultiplexes a part of the optical signal amplified by the EDF 111 to use a part of the demultiplexed optical signal as an output optical signal, while sending the other part of the demultiplexed optical signal as monitor light to the optical/electric converting unit 108. The optical/electric converting unit 108 converts the optical signal sent from the second demultiplexer 114 into an electric signal (voltage signal), similarly to the optical/electric converting unit 101. Namely, the optical/electric converting unit 108 converts the optical signal demultiplexed by the second demultiplexer 114 into a voltage signal proportional to an optical output level of the optical signal, and sends the voltage signal as an output monitor signal to the optical output constant control unit 107.
The optical output constant control unit 107 controls the pumping source driving unit 105 on the basis of the output monitor signal from the optical/electric converting unit 108 such that a level (optical output level) of the output optical signal is stable. In concrete, the optical output constant control unit 107 compares the output monitor signal (voltage signal) inputted from the optical/electric converting unit 108 with a reference voltage value corresponding to a desired optical output set in advance, and controls the pumping source driving unit 105 such that pump light corresponding to a difference in voltage between the reference voltage value and the output monitor signal is outputted from the pumping source 104.
In the above structure, a part of an optical signal inputted to the apparatus through the optical fiber is demultiplexed by the first demultiplexer 109, sent to the EDF 111 through the isolator 110, and amplified with pump light inputted from the multiplexer 112 in the EDF 111. The amplified optical signal is sent to the second demultiplexer 114 through the isolator 113, and a part of the amplified optical signal is demultiplexed by the second demultiplexer 114, then outputted as an amplified optical signal (output light signal).
On the other hand, the part of the input light demultiplexed by the first demultiplexer 109 is sent to the optical/electric converting unit 101, and converted into a voltage signal proportional to an optical level of the input light in the optical/electric converting unit 101. The optical signal converted into the voltage signal is sent as an input monitor signal to the input disconnection detecting unit 102. The input disconnection detecting unit 102 compares a level of the input monitor signal with a reference value set in advance to detect whether the input light is disconnected or not.
In a state where the input disconnection detecting unit 102 does not detect input disconnection and an optical signal above a predetermined level is inputted to the inputting side for the purpose of optical amplification, that is, in a state where an optical signal to be inputted is in a steady state condition, the operation sequence unit 103 controls switching of the operation change-over switch 106 such that a control signal from the optical output constant control unit 107 is outputted to the pumping source driving unit 105.
In this case, a part of the amplified optical signal demultiplexed by the second demultiplexer 114 is converted into an output monitor signal which is an electric signal (voltage signal) by the optical/electric converting unit 108, and the output monitor signal is sent to the optical output constant control unit 107. The optical output constant control unit 107 compares the output monitor signal with a predetermined reference voltage value, and controls the pumping source driving unit 105 such that pump light at a level corresponding to a difference in voltage between the output monitor signal and the reference voltage value is outputted from the pumping source 104. Under the control of the optical output constant control unit 107, the pumping source driving unit 105 drives the pumping source 104 to supply pump light so that the EDF 111 obtains desired amplified light.
If the input monitor signal drops below the reference value when the input disconnection detecting unit 102 compares the input monitor signal with the reference value set in advance, the input disconnection detecting unit 102 determines that the input light is disconnected, and sends a signal notifying of disconnection of the input light to the operation sequence unit 103.
The operation sequence unit 103 notified of disconnection of the input light controls the operation change-over switch 106 in order to halt the optical output constant control to selectively switches a change-over switch in the operation change-over switch 106 from a side of the optical outputting control unit 107 to a side of ground, thereby grounding the pumping source driving unit 105. The pumping source driving units 105 halts the driving of the pumping source 104 by being grounded, so that inputting of the pump light from the pumping source 104 to the EDF 111 is stopped.
When power is again on or the input optical signal is recovered, the power-on/recovery detecting unit not shown sends a signal notifying of that to the operation sequence unit 103. The operation sequence unit 103 controls the operation change-over switch 106 to switch the change-over switch to the optical output constant control unit 107, thereby again performing the optical output constant control.
States of each of light at the time of power-on or input light recovery in the optical amplifying apparatus will be next described with reference to FIGS. 10(a) through 10(c). FIG. 10(a) is a diagram showing with time changes of input light at the time of power-on and input light recovery in the optical amplifying apparatus, FIG. 10(b) is a diagram showing with time changes of pump light at that time, and FIG. 10(c) is a diagram showing with time changes of output light at that time.
As soon as power of the optical amplifying apparatus is on and an optical signal is inputted to the optical amplifying apparatus [refer to A1 in FIG. 10(a)], pump light corresponding to a level of the input light is driven [refer to A2 in FIG. 10(b)], and the pump light is increased while generating a delay due to a transmission resistance of the circuit of the optical amplifying apparatus and the like [refer to A2 to A3 in FIG. 10(b)]. A little after an optical signal is inputted, outputting of the amplified optical signal is initiated [refer to A4 in FIG. 10(c)].
When the input light is disconnected [refer to A5 in FIG. 10(a)], the above input disconnection detecting unit 102 detects the input disconnection, outputting of the pump light is stopped by the operation sequence unit 103 and the operation change-over switch 106 [refer to A6 in FIG. 10(b)], thus optical output is also stopped [refer to A7 in FIG. 10(c)].
If an optical signal [refer to A8 in FIG. 10(a)] stronger than the optical light inputted the last time [refer to A1 in FIG. 10(a)] is inputted when the input light is recovered, pump light corresponding to a level of the input light is driven [refer to A9 in FIG. 10(b)]. The pump light is increased while generating a delay due to a transmission resistance of the circuit of the optical amplifying apparatus and the like [refer to A9 to A10 in FIG. 10(b)], then outputting of an amplified optical signal is initiated a little after the optical signal is inputted [refer to A11 in FIG. 10(c)].
However, in the case of the output constant control in the above optical amplifying apparatus, the input disconnection detecting unit 102 detects disconnection of input light, while the optical output constant control unit 107 controls the pumping source driving unit 105 to output pump light corresponding to the disconnected input light.
Namely, pump light is abruptly increased when power is on or input light is recovered in the optical amplifying apparatus [refer to A3 and A10 in FIG. 10(b)], and output light amplified by the pump light jumps up above a predetermined level for a moment before the output light is stabilized at a predetermined level [refer to A12 and A13 in FIG. 10(c)].
When such protruding output light is generated in the optical amplifier used in an optical transmission system, the generated protruding output light (excessive output) is accumulatively amplified while passing through optical transmission paths, other repeaters and the like, the excessive output light is further increased when the terminal receives the optical signal, which exerts an effect on qualities of optical components receiving the optical signal in the receiving unit.
In order to prevent pump light output from being abruptly increased and output light from being excessive, there has been a technique of decreasing a responsiveness of a circuit performing the optical amplifier output constant control. However, when a speed of the optical amplifier output constant control is decreased, the optical output constant function of the optical amplifier is degraded, such that fluctuation in level of the input light is prone to appear in the output light.
There has been also an optical output controlling technique by the optical amplifying apparatus performing a pump light output constant control to keep light output from the pumping source constant without performing the output constant control immediately after power is on or input light is recovered in the optical amplifier, or a current constant control to keep a driving current driving a pumping source constant.
Now, another optical amplifying apparatus using a pump light output constant control will be described with reference to FIG. 11. FIG. 11 is a diagram schematically showing a structure of another optical amplifying apparatus using a pump light output constant control.
The optical amplifying apparatus using a pump light output constant control has, as shown in FIG. 11, a timer 115 and a pump light control unit 116, additionally to the optical amplifying apparatus shown in FIG. 9. Further, the optical amplifying apparatus has an operation sequence unit 118 instead of the operation sequence unit 103 and an operation change-over switch 117 instead of the operation change-over switch 106. The other parts of the optical amplifying apparatus are similar to those of the optical amplifying apparatus shown in FIG. 9. Incidentally, like reference characters in the drawing designate like or corresponding parts, descriptions of which are thus omitted.
The operation sequence unit 118 controls an operation of the operation change-over switch 117. When inputted thereto a signal notifying that input light is disconnected from the input disconnection detecting unit 102, the operation sequence unit 118 controls the operation change-over switch 117 to halt the optical output constant control.
A power-on/recovery detecting unit not shown for detecting that power is on or that the input optical signal is recovered is connected to the operation sequence unit 118. When power is on or the input optical signal is recovered, the power-on/recovery detecting unit sends a signal notifying that power-on/recovery is detected to the operation sequence unit 118.
The timer 115 is connected to the operation sequence unit 118. The timer 115 sends a trigger signal indicating that counting is terminated to the operation sequence unit 118 after a predetermined period set in advance is elapsed from when the timer 115 receives a signal indicating initiation of counting from the operation sequence unit 118.
When the power-on/recovery detecting unit detects that power is on or that an input optical signal is recovered, the operation sequence unit 118 sends a trigger signal indicating initiation of counting to the timer 115, while controlling the operation change-over switch 117 to perform the pump light control until receiving a trigger signal indicating termination of the counting from the timer 115.
An end of the operation change-over switch 117 is connected to the pumping source driving unit 105, while the other end of the same is switchably connected to any one of the optical output constant control unit 107, the pump light control unit 116 or ground, thereby being selectively switched under a control of the operation sequence unit 118. The other end of the operation change-over switch 117 is connected to the optical output constant control unit 107 when an inputted optical signal is in a steady state condition, whereas being grounded when the operation sequence unit 118 is notified that the inputted optical signal is disconnected.
An end of the pump light control unit 116 is connected to the pumping source 104 to monitor pump light outputted from the pumping source 104. The other end of the pump light control unit 116 is connected to the operation change-over switch 117 to be selectively connected to the pumping source driving unit 105 by a change-over switch in the operation change-over switch 117, so as to perform the pump light output constant control by controlling the pumping source driving unit 105 such that a quantity of pump light outputted from the pumping source 104 is stabilized at a predetermined value set in advance.
The optical amplifying apparatus using the pump light output constant control with the above structure amplifies an optical signal inputted thereto through an optical fiber and outputs the optical signal, and performs the optical output constant control by the optical output constant control unit 107 on the basis of an optical signal (output monitor signal) demultiplexed by the second demultiplexer 114, similarly to the optical amplifying apparatus shown in FIG. 9. When an input of optical signal is disconnected, the operation sequence unit 118 controls the operation change-over switch 117 on the basis of an optical signal (input monitor signal) demultiplexed by the first demultiplexer 109 to connect the pumping source driving unit 105 to ground in order to halt the optical output constant control, thereby stopping inputting of pump light from the pumping source 104 to the EDF 111.
In the optical amplifying apparatus using the pump light output constant control shown in FIG. 11, when the operation sequence unit 118 is notified from the power-on/recovery detecting unit (not shown) that power is on or input light is recovered in the optical amplifying apparatus, the operation sequence unit 118 controls switching of the operation change-over switch 117 such as to shift a state of the pumping source driving unit 105 from an operation stop state (grounded) to a pump light constant control state by the pump light control unit 116. Further, the timer 115 starts to count with the above power-on/recovery detection information as trigger information.
The operation sequence unit 118 controls the operation change-over switch 117 on the basis of counting by the timer 115 such that the pump light control unit 116 performs the pump light output constant control until a predetermined time is elapsed When the predetermined time is elapsed, the operation sequence unit 118 controls switching of the operation change-over switch 117 to switch from the pump light constant control by the pump light controlling unit 116 to the optical output constant control by the optical output constant control unit 107.
Now, states of each light at the time of power-on and input light recovery in the other optical amplifying apparatus using the pump light output constant control will be described with reference to FIGS. 12(a) through 12(c). FIG. 12(a) is a diagram showing with time changes of input light at the time of power-on and input light recovery, FIG. 12(b) is a diagram showing with time changes of pump light at that time, and FIG. 12(c) is a diagram showing with time changes of output light at that time.
In the above optical amplifying apparatus shown in FIG. 11, when power is on and an optical signal is inputted [refer to B1 in FIG. 12(a)], the timer 115 starts to count, and the operation sequence unit 118 controls switching of the operation change-over switch 117 on the basis of counting by the timer 115 such that the pumping source driving unit 105 is connected to the pump light control unit 116 to perform the pump light output constant control until a predetermined time is elapsed [refer to B2 in FIG. 12(b)]. An optical signal amplified with the pump light is outputted [refer to B6 in FIG. 12(c)].
When the counting by the timer 115 is terminated after the predetermined time is elapsed [refer to B3 in FIG. 12(b)], the operation sequence unit 118 controls switching of the operation change-over switch 117 to connected the pumping source driving unit 105 to the optical output constant control unit 107 so as to perform the optical output constant control [refer to B4 in FIG. 12(b)]. Outputting of output light amplified with the pump light is initiated a little after the control is switched to the above optical output constant control [refer to B7 in FIG. 12(c)].
When the input light is disconnected [refer to B5 in FIG. 12(a)], the above input disconnection detecting unit 102 detects input disconnection, the operation sequence unit 118 controls switching of the operation change-over switch 117 to connect the pumping source driving unit 105 to ground, outputting of the pump light from the pumping source 104 to the EDF 111 is thereby stopped [refer to B8 in FIG. 12(b)], thus optical output is also stopped [refer to B9 in FIG. 12(c)].
When the input light is recovered and an optical signal stronger than the input light inputted the last time is inputted [refer to B10 in FIG. 12(a)], pump light corresponding to a level of the input light is driven [refer to B12 in FIG. 12(b)], then outputting of output light amplified with the pump light is initiated a little after that [refer to B13 in FIG. 10(c)].
When the output constant control is performed in the optical amplifying apparatus using the pump light constant control shown in FIG. 11, gain characteristics of the optical amplifier is approximately proportional to an energy of pump light outputted from the pumping source. For this, if a quantity of pump light or a pumping source driving current is so set in the pump light control unit 116 that a predetermined optical output is obtained even when input light is small, the optical output might exceed a predetermined optical input level when greater input light is inputted.
Accordingly, it is necessary to set the pump light output or the pumping source driving current in the pump light control unit 116 such that the optical output does not exceed the predetermined optical output even when the input light is at maximum. If the pump light control unit 116 is set as above, a predetermined optical output cannot be obtained when input light is small, and pump light output is abruptly increased when the counting by the timer 115 is completed and the optical output constant control is initiated [refer to B14 in FIG. 12(b)], and the optical output thus exceeds the predetermined optical input level [refer to B15 in FIG. 12(c)].
When the protruding output light is generated in the optical amplifying apparatus used in an optical transmission system, the generated protruding output (excessive output) light is accumulatively amplified while passing through transmission paths, other repeaters and the like, the excessive output light is further increased when the terminal receives the optical signal, the excessive output light thus affects on qualities of optical components receiving the optical signal in the receiving unit, as stated above.